Devirtualize Document class type checking

Source/core/html/ImageDocument.h

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 33 matching lines @@
     void disconnectImageElement() { m_imageElement = 0; }
     
     void windowSizeChanged();
     void imageUpdated();
     void imageClicked(int x, int y);
 
 private:
     ImageDocument(Frame*, const KURL&);
 
     virtual PassRefPtr<DocumentParser> createParser();
-    virtual bool isImageDocument() const { return true; }
-    
+
     void createDocumentStructure();
     void resizeImageToFit();
     void restoreImageSize();
     bool imageFitsInWindow() const;
     bool shouldShrinkToFit() const;
     float scale() const;
     
     ImageDocumentElement* m_imageElement;
     
     // Whether enough of the image has been loaded to determine its size
@@ skipping 17 matching lines @@
     ASSERT_WITH_SECURITY_IMPLICATION(!document || document->isImageDocument());
     return static_cast<const ImageDocument*>(document);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toImageDocument(const ImageDocument*);
 
 }
 
 #endif // ImageDocument_h